Mirror changes to side of cube across all six sides
Modifiers:
You could do this with a Mirror modifier and an Array Modifier:

Delete the excess geometry:


Add a Mirror modifier:


Add an Array Modifier.

Add an Empty rotated -90°, 0°, 90° (assuming the mirror is mirroring along the X axis):


Set the empty as the Object offset in the array modifier:



Result:

Duplifaces:
Another way is with Duplifaces:

Add a cube and a plane.

Parent the plane to the cube (CtrlP):


With the cube selected, enable Duplifaces in the Properties Editor &gt; Object &gt; Duplication:


Edit the plane object:



The duplifaces will be displayed in object mode:

Limitations:

The array modifier method will not rotate the duplicated faces in the same direction: (this is tweak-able by adjusting the rotation of the empty)


Duplifaces do not display in edit mode.